Logic circuits are basic building blocks in digital electronics and process information encoded as two-valued voltage or current levels, usually denoted as 0 and 1. There are three fundamental logic functions: conjunction (AND), disjunction (OR) and negation (NOT). These fundamental functions can be combined in various ways, and most commonly, as NOT with AND and OR to form NAND and NOR circuits from which all combinational logic functions can be realized in integrated circuit (IC) form. Combinational logic is that logic whose output is determined solely by the current inputs, as distinguished from sequential logic which operates on both prior and current inputs. Relationships among combined logic elements are defined by the rules of Boolean algebra.
Among the operations susceptible of implementation by logic circuits are the arithmetic functions of addition, subtraction, multiplication and division. In an arithmetic adder, for example, a combinational network of logic elements is interconnected to generate the sum digit as an output by monitoring combinations of input digits. An output 1 digit is generated only in response to those input combinations set forth in an addition table in a so-called half-adder arrangement. The combinational network can be expanded to generate and operate on carry digits in a full adder arrangement.
A well known full adder is implemented by an array of gates with five NOR gates and three NOT gates in the sum portion and four NOR gates in the carry portion and in its IC form is equivalent to forth metal-oxide semiconductor (MOS) transistors.
It is an object of this invention to provide a more economical full adder using fewer circuit elements and less power than conventional all-NOR gate adders.